Process for purifying lactams

ABSTRACT

A process is described for purifying lactams, in particular the lactams obtained by cyclizing hydrolysis of an aminoalkylnitrile, and more particularly to the purification by crystallization of ε-caprolactam obtained by cyclizing hydrolysis of aminocapronitrile. This crystallization is carried out on the cyclization reaction medium after hydrogenation, removal of volatile compounds, and treatment with an ion-exchange resin.

This application is a 371 of PCT/FR00/03694, filed Dec. 27, 2000,published in French as WO 01/49665.

The invention relates to a process for purifying lactams, in particularthe lactams obtained by cyclizing hydrolysis of an aminoalkylnitrile.

The invention relates more particularly to the purification ofε-caprolactam obtained by cyclizing hydrolysis of aminocapronitrile.

Lactams and in particular ε-caprolactam are important chemical compoundsused as starting materials in the manufacture of many products. One ofthe most important applications is the manufacture of polymers such aspolyamides.

Among the processes for synthesizing lactams and in particularε-caprolactam which have been proposed is a process consisting inmanufacturing an aminoalkylnitrile such as aminocapronitrile and then incarrying out a cyclizing hydrolysis of this aminonitrile. This process,which may be carried out in liquid or gaseous medium with or withoutsolvent, is disclosed, for example, in patents EP 0 729 454, EP 0 729453, EP 0 793 650, EP 0 793 651, EP 0 794 943, EP 0 815 077, EP 0 815078, EP 0 748 797, EP 0 6559 741 and WO 96/22974.

An important step in the manufacture of lactams, in particularε-caprolactam, when these need to be used as monomers in the manufactureof polymers such as polyamides, is the purification of the products.Specifically, for these applications, the lactam must satisfy highlystringent and demanding purity criteria. These criteria are defined byvarious indices or concentrations corresponding to several types orcategories of impurity. These main criteria are:

permanganate number (I_(KMnO) ₄ )

content of volatile bases (I_(VB))

UV index (I_(UV))

Various purification processes have already been proposed. Thus, U.S.Pat. No. 5,496,941 discloses a process for purifying caprolactamextracted from the reaction medium of a cyclizing hydrolysis bydistillation comprising a hydrogenation step followed by a treatment inacidic medium (treatment on ion-exchange resin or distillation insulphuric acid medium) and a distillation in basic medium.

European patent application No. 94/3608 discloses a process forpurifying crude ε-caprolactam obtained by cyclization of an alkyl6-aminocaproate, which consists in carrying out a crystallization of thecaprolactam. According to the said document, certain components such asN-methylcaprolactam, methylvalerolactam and valeramide are removed bycrystallization. The process disclosed indicates that the caprolactamthus crystallized satisfies the required purity criteria regarding thecontent of volatile bases and the UV index for absorption at awavelength of 290 nm.

However, the said document does not specify whether or not the thirdpurity criterion, the permanganate number, is achieved.

In the case of a caprolactam obtained from the cyclizing hydrolysis ofan aminocapronitrile obtained by semihydrogenation of adiponitrile, thepurity criteria of the caprolactam are achieved by carrying outpurification processes described above comprising several successivetreatments, in particular an acidic treatment and a final distillationin basic medium.

In such processes, the production of a caprolactam of the desired purityrequires the implementation of a distillation operation with a largeheavy fraction (fraction of products with a high boiling point)comprising a large amount of lactam in the form of lactam or oligomers.This loss of lactam is detrimental to the process.

One of the aims of the present invention is especially to overcome thisdrawback by proposing a process for purifying lactams making it possibleto obtain a product that satisfies the desired purity criteria, thuslimiting the lactam treatment steps and more particularly those whichmay generate the formation of oligomers.

To this end, the invention proposes a process for purifying a lactamobtained from the cyclization of an aminoalkylnitrile obtained bysemihydrogenation of an alkyldinitrile, which consists, after removingthe volatile compounds from the cyclization medium, in extracting thelactam formed by crystallization.

According to another preferred characteristic of the invention, thecrystalline lactam can be subjected to one or more other crystallizationsteps to obtain the desired purity. Other purification treatments may becarried out on the crystalline lactam without departing from the scopeof the present invention.

According to another embodiment, the cyclization medium may be subjectedto a hydrogenation before the step to remove the volatile compounds

According to yet another embodiment, the crystallization of thecaprolactam is carried out on the cyclization medium which may or maynot have undergone a hydrogenation, after treating this medium in acidmedium, for example by passing it through an ion-exchange resin or adistillation in acidic medium.

According to another embodiment, a hydrogenation of the cyclizationmedium may be carried out after removing tie volatile compounds.

For simplicity, the expression “cyclization medium” for carrying out thecrystallization means either the reaction medium obtained from thecyclization step on which the various treatments described above arecarried out, or the medium formed by the caprolactam extracted after thecyclization reaction, for example by distillation or entrainment, inother words after removal or separation of the volatile compounds andthe high-boiling compounds. The classification into volatile compoundsand high-boiling compounds is made relative to the boiling point of thelactam to be recovered.

According to the invention, the extraction and recovery of the lactam bya crystallization make it possible to obtain a product which satisfiesthe desired purity criteria, without the need for treatment attemperatures which may generate the formation of oligomers, for example.

Thus, the degradation or polymerization of the lactam, which arises whenit is heated to high temperatures, especially in order to distil it, isavoided.

The process of the invention applies more particularly to thepurification of ε-caprolactam obtained by cyclizing hydrolysis ofaminocapronitrile.

This aminocapronitrile is synthesized by semihydrogenation ofadiponitrile. These various semihydrogenation processes are disclosed,for example, in patents EP 0 737 100, EP 0 737 181 and WO 96/18603.

According to the invention, the solvents that are suitable for thecrystallization may be a lactam, water, saturated hydrocarbons such ashexane or cyclohexane, alcohols, aromatic hydrocarbons, organohalogencompounds such as CCl₄ or CHCl₃, ketones or the like.

Solvents that well preferably be used include water and/or a lactam andmore preferably a lactam which is identical to the one which needs to bepurified, or, more preferably, a saturated aqueous lactam solution.

The crystallization may be carried out in one step or in several steps.The mother liquor obtained in one step is advantageously recycled intoone of the preceding steps. More generally, the crystallization may becarried out according to the conventional industrial crystallizationtechniques.

According to another characteristic of the process of the invention, thecompound obtained after crystallization is filtered off, spin-filteredor isolated by any other common solid/liquid separation technique. Thesolid recovered may be advantageously washed with a solvent, preferablywith a saturated lactam solution.

Advantageously, the medium containing the lactam to be crystallized isconcentrated by any means which is suitable to obtain a weightconcentration of lactam of greater than 80% by weight, preferablygreater than 90%.

Thus, examples of techniques for saturating or concentrating thesolution which may be mentioned include techniques of cooling andevaporation advantageously under reduced pressure to work at lowertemperature.

In addition, the crystallization can be performed with seeding of thesolution in order to promote nucleation of the lactam.

The pure lactam recovered after solid/liquid separation, for examplefiltration, and optional washing, is dried or the solvent, for examplewater, is distilled off according to usual techniques (when the solventis water, the caprolactam is dehydrated).

The process of the invention thus makes it possible to produce a lactam,more particularly an ε-caprolactam, which has the desired puritycriteria for use as monomer in the manufacture of polyamides such asPA6. The polyamides thus obtained are suitable in particular for themanufacture of textile yarns or fibres with working properties that areequivalent to those of the current polyamides.

Other advantages and details of the invention will be illustrated by theimplementation examples below, given purely as a guide.

EXAMPLE 1

An ε-caprolactam is manufactured according to the process disclosed inthe patents cited above.

Thus, this ε-caprolactam is obtained according to Example 1 of patent EP0 748 797 by semihydrogenation of adiponitrile followed by cyclizinghydrolysis of aminocapronitrile according to the conditions described inthe said example.

The medium obtained is subjected to a hydrogenation according to theprocess disclosed in the as yet unpublished french patent applicationfiled under No. 98/14735 on Nov. 19, 1998.

The ammonia present in the hydrogenated cyclizing hydrolysis reactionmedium is then removed by evaporation or entrainment.

The cyclization medium thus obtained is then treated by passing itthrough an ion-exchange resin according to the conditions disclosed inthe patents or patent applications WO 98/05636 or WO 96/20923.

The medium obtained after treatment with resin contains 65% by weight ofε-caprolactam and numerous other products which were not all identified.

The medium thus recovered is analysed to determine the purity criteriadefined above. The results of these analyses are indicated in the tablebelow.

In accordance with the process of the invention, the caprolactam isrecovered by crystallization from this solution after concentrationunder reduced pressure at a caprolactam content in the region of 90% byweight.

The solution is concentrated at a temperature of 40° C. and is thencooled to a temperature in the region of 20° C. at a cooling rate ofabout 10° C. per hour.

The crystals formed are recovered by filtration on a sinter funnel. Themother liquors recovered are stored for possible recycling.

The sold cake of caprolactam crystals is washed with a saturated aqueouscaprolactam solution. The degree of washing with wet base isadvantageously between 1 and 5.

The crystalline caprolactam is analysed. The purity characteristics arecollated in Table I below.

Test Medium I_(UV) I_(KMO4) I_(VB) (meq/kg) Medium before 0.45 32.062.69 crystallization Wet cake after 0.02 2.03 0.04 washing

EXAMPLE 2

A caprolactam is manufactured according to the process described inExample 1 and with reference to the process disclosed in patentapplication EP 0 748 797.

However, in this example, the medium obtained from the cyclizinghydrolysis is only treated to remove the volatile compounds, moreparticularly ammonia.

The crystallization of the caprolactam formed is carried out directlyusing this solution after concentration by evaporation of water.

The crystallization is carried out in two successive steps. The cakerecovered in the first step is taken up to give a new saturated aqueoussolution and is then recrystallized. The purity characteristics of thecake recrystallized after washing are given in Table II below.

Test Medium I_(UV) I_(KMO4) I_(VB) (meq/kg) 2 Medium before 17.7 400 130crystallization Wet cake 0.03 2.0 0.20 recrystallized after washing

1. A process for purifying lactams from a cyclization medium obtained bycyclization of aminoalkylnitriles, without a treatment at hightemperatures which generates degradation or polymerization of saidlactams which comprises: hydrogenating a cyclization medium obtainedfrom the cyclization reaction; removing volatile compounds from thecyclization medium; treating the cyclization medium with an ion-exchangeresin; and recovering the lactam by crystallization from the cyclizationmedium, wherein crystallization is effected by seeding to promotenucleation of the lactam; wherein the steps of hydrogenating acyclization medium obtained from the cyclization reaction, removingvolatile compounds from the cyclization medium, and treating thecyclization medium with an ion-exchange resin, are conducted prior tocrystallization of the cyclization medium.
 2. Process according to claim1, wherein the crystallization is carried out in several steps. 3.Process according to claim 1, wherein the hydrogenation occurs beforeremoving the volatile compounds.
 4. Process according to claim 1,wherein the hydrogenation occurs after separation of the volatilecompounds.
 5. Process according to claim 1, wherein the lactam isε-caprolactam.
 6. Process according to claim 5, wherein theε-caprolactam is produced by cyclizing hydrolysis of aminocapronitrile.7. Process according to claim 6, wherein the aminocapronitrile issynthesized by semihydrogenation of adiponitrile.
 8. Process accordingto claim 1, wherein a solvent is used for crystallization and isselected from the group consisting of a lactam, an aqueous lactamsolution, water, saturated hydrocarbons, alcohols, aromatichydrocarbons, organohalogen compounds and ketones.
 9. Process accordingto claim 8, wherein the solvent is a saturated aqueous lactam solution.10. Process according to claim 1, wherein the weight concentration oflactam in the cyclization medium to be crystallized is greater than 80%by weight.
 11. The process according to claim 1, wherein the lactamobtained after crystallization is filtered off, spin-filtered orisolated by solid/liquid separation.